In the usual preparation of lower fatty acid esters of cellulose, sulfuric acid is the catalyst employed. This acid is very effective in that it promotes the desired esterification of the cellulose under relatively mild conditions but it combines with cellulose and the cellulose ester obtained often contains sulfuric acid radicals. Ordinarily these sulfate groups can be removed from the final product by a prolonged hydrolysis under very carefully controlled conditions. However, even after this treatment, the product may contain sufficient combined sulfuric acid to render it unstable to prolonged heating at elevated temperatures such as may be encountered in molding operations or the like.
In order to eliminate this disadvantage which may be encountered when sulfuric acid catalyst is employed, various so-called "non-combining" acids have been suggested as catalysts in cellulose esterification processes. Perchloric acid is an example of such an acid catalyst. However, perchloric acid has several disadvantages when used as a catalyst. For instance it is corrosive on stainless steel equipment. It is explosive in contact with readily oxidizable materials.
According to U.S. Pat. No. 2,861,069, other strong acid catalysts such as phosphoric acid, hydrochloric acid, methane sulfonic acid, toluene sulfonic acid and the like have also been disclosed as useful noncombining catalysts in the esterification of cellulose. In addition, weak salts of strong acids such as zinc chloride, ammonium sulfates and the like have been suggested.
Commercial cellulose esterifications, for the most part, are consequently conducted using relatively large quantities of sulfuric acid as the reaction catalyst. Sulfuric acid is a very strong acid catalyst for cellulose esterification and requires considerable reaction temperature manipulation (cooling) in order to avoid excessive degradation (reduction) of the cellulose molecular weight. Sulfuric acid also combines with the cellulose during esterification to form a cellulose sulfate ester which decreases the thermal stability of the cellulose organic ester. Other cellulose esterification catalysts such as perchloric acid, hydrochloric acid, and zinc chloride are well known in the art as noncombining catalysts. However, these catalysts are either too corrosive (perchloric and hydrochloric acid) or too much (50 to 100% based on weight of celluose) is required for efficient reaction (zinc chloride).
Certain sulfonic acids, e.g., para-toluene sulfonic acid, also are known in the art as cellulose esterification catalysts but again large amounts (up to 50 wt. % based on cellulose) of catalyst are required at the normal reaction temperature (40.degree.-50.degree. C.) and high quality, high .alpha.-cellulose content pulps are required to obtain an acceptable quality product.
The present invention is believed to be the first realization that very small amounts of a catalyst mixture comprised of sulfuric and phosphoric acids may be used at reaction temperatures of 75.degree.-110.degree. C. to obtain acceptable quality cellulose acetate from low .alpha.-cellulose (e.g., viscose grade) wood pulps.